1. Field of the Invention
This invention pertains to a method for upgrading black oil and, in particular to a method for removing ash, asphaltenic, metallic and sulfurous contaminants from a black oil by contacting said oil with a liquid paraffinic solvent in the presence of water.
2. Description of the Prior Art
Heavy hydrocarbon fractions like atmospheric tower bottoms products, vacuum tower bottoms products (vacuum residuum), shale oils, coal oils and tar sands are commonly referred to in the art as "black oils" and contain appreciable amounts of asphaltenic material. Asphaltenes are present in black oils in a colloidal state. The high molecular weight asphaltenes are generally complexed or linked with sulfur and organometallic contaminants. In addition, many black oils contain finely divided particulate ash. The ash consists principally of alumino-silicates having nominal diameters in the range of about 1.0 to about 10.0 microns.
Such contaminants in black oils rapidly poison the catalysts which are generally employed in catalytic processes such as catalytic cracking and hydroprocessing and have a strong corrosive effect on metallic equipment and cause damage to refractory materials. In addition, the finely divided ash particles have extremely sharp edges and are, therefore, excessively abrasive. Because of these deleterious characteristics, a variety of processes have been employed to remove such contaminants from black oils.
Deasphalting is one such technique for separating such contaminants from the non-asphaltenic fraction of a black oil. The resulting deasphalted fraction contains only a small portion of the original ash, metallic and sulfurous contaminants and virtually no asphaltenes. Deasphalting improves the quality of a heavy hydrocarbon fraction by lowering the viscosity, by reducing the ash, metals and sulfur content, and by rejecting the asphalt fraction.
In deasphalting, a black oil feed is contacted with a relatively large volume of a deasphalting solvent, generally in a counter-current contactor with the inlet for the oil feed being located above the inlet for the solvent. Upon contact of the solvent with the oil feed, the colloidal state of the asphaltenes is broken down, causing the asphaltenes to flocculate and settle out of the oil phase, taking much of the ash, metallic and sulfurous contaminants with them; and a second phase is formed which contains substantially all of the asphaltenes and a substantial proportion of the ash, metallic, and sulfurous contaminants. A deasphalted hydrocarbon or oil phase exits from the top region of the contactor, and a predominantly asphaltene phase exits from the bottom region of the contactor. The solvent is next flashed and stripped from both the deasphalted hydrocarbon phase and the asphaltene phase and then condensed and returned to the contactor.
A deasphalting solvent is one in which the asphaltenic materials are relatively insoluble but which is miscible with the hydrocarbon components of the black oil. Typically, liquefied propane is used as a deasphalting solvent. However, propane is a poor solvent for resins as well as for asphaltenes, and thus a propane solvent rejects large amounts of useful hydrocarbons as well as the asphaltenes. Consequently, the yield of useful deasphalted hydrocarbons is reduced when propane is the deasphalting solvent.
Higher molecular weight deasphalting solvents, such as pentane, hexane, and heptane, are better solvents for resins than is propane, and their use in deasphalting results in improved yields of useful deasphalted hydrocarbons. Such higher molecular weight deasphalting solvents reject the asphaltenic contaminants selectively. However, use of these higher molecular solvents results in slower settling of the asphaltenes out of the oil phase. Further, a major difficulty in deasphalting is the large volume of deasphalting solvent which must be employed. It is desirable to reduce the amount of deasphalting solvent required for effective deasphalting operations and to increase the ease and speed of settling out of the flocculated asphaltenes from the oil phase.
It has been unexpectedly found that deasphalting with a higher molecular weight solvent in the presence of added fresh water permits the weight ratio of deasphalting solvent-to-black oil in deasphalting operations to be reduced markedly without affecting the yield or quality of the deasphalted hydrocarbon product and results in more rapid separation of flocculated asphaltenes from the oil phase.
Malikow, et al., German Offenlegungsschrift No. 2,260,777 (June 20, 1974) discloses a method for deasphalting heavy petroleum residues which is similar to the method of this invention. The method of Malikow, et al., involves a liquid-liquid extraction of the heavy petroleum residue with a solvent made up of paraffinic hydrocarbons containing from 4 to 8 carbon atoms, at a pressure higher than the vapor pressure of the solvent, at a temperature between 100.degree. C. and the critical boiling temperature of the solvent, and in the presence of water in an amount of at least 2 volume percent of the heavy petroleum residue.